In this file photo, a man exhales vapor from an e-cigarette in New York. A study at 10 Los Angeles high schools links e-cigarettes with later tobacco use. The government-funded study was published Tuesday, Aug. 18, 2015 in the Journal of the American Medical Association. It doesn’t prove that electronic cigarettes are a “gateway drug” but some doctors say it bolsters arguments that the devices need to be strictly regulated.

Researchers with USC and other universities found that teens who try e-cigarettes are more likely to turn to tobacco products than those who have never tried vaping.

The study, published Tuesday in the Journal of the American Medical Association, shows that 31 percent of the 222 youths who had tried vaping turned to cigarettes, cigars and hookahs six months after they first were surveyed. Among those who never tried vaping, 8 percent had tried tobacco products in the same time period.

Researchers surveyed 2,530 teens, all 14-year-olds from nearly a dozen Los Angeles area high schools, including in the San Gabriel and San Fernando valleys, over a yearlong period ending last fall.

It’s unclear why some teens who vape turn to tobacco, said Adam Leventhal, an associate professor and director of the USC Health, Emotion and Addiction Laboratory at the Keck School of Medicine who led the study.

E-cigarettes are battery-operated devices that deliver nicotine with flavorings — and that could be the answer, he said.

“One of our hypotheses is nicotine is highly addictive,” Leventhal said. “Among teens, a significant portion find the experience pleasurable, and so they might be more open to trying tobacco.”

He also said the results show that when looking at how to design policies to protect public health, a distinction should be made between adults who are smokers who use e-cigarettes for tobacco reduction and teens who vape for recreation.

State bills that propose regulating e-cigarettes as a tobacco product are expected to be discussed Wednesday by lawmakers during a special session in Sacramento.

“E-cigarettes may be drawing a new generation of teens into recreational nicotine use because they are high-tech, can be purchased somewhat easily, come in enticing flavors and have a perception that they’re not harmful,” he said. “Some e-cigarette devices appear to be very efficient at delivering nicotine to the lungs and brain.”

Leventhal said researchers accounted for some risk factors such as friends who smoke, but not all, a fact that the American Vaping Association grasped as to why the results should be interpreted with caution.

“Even though teen experimentation with vapor products is tied to the most dramatic decline in youth smoking in U.S. history, there appears no shortage of politically driven commentators who are eager to paint vaping as a gateway to smoking,” said Gregory Conley, president of the association. “Unfortunately, ardent opponents of vaping have a history of making wild and unsubstantiated claims, and this study will be twisted and used to push for unjustified excise taxes and small-business-killing regulations.”

Conley said since e-cigarettes were introduced 10 years ago, the products have helped millions of adults quit smoking.

But keeping teens away is proving difficult.

Earlier this year, the California Department of Public Health declared liquid nicotine projects a public health threat due to the escalated use of e-cigarettes among teens and young adults. Researchers found that more than twice as many 8th- and 10th-graders reported vaping than smoking traditional cigarettes in 2014. Among seniors in high school, more than 17 percent said they used e-cigarettes compared with 14 percent who smoked traditional cigarettes.

The study is believed to be a first of its kind and was funded by the federal Food and Drug Administration through the National Institutes of Health. In July, the FDA issued a brief on e-cigarettes but had no answers on the health risks of the devices. Because the products have not been fully studied, consumers don’t know the potential risks, how much nicotine or other chemicals are being inhaled and whether there are any benefits, according to the agency’s brief.

Leventhal said he believes the study is important because it coincides with the national phenomenon of teens and vaping.

“I think restrictions against use (among teens) is an important positive step,” Leventhal said. “Another potential question is the oversight of types of flavors and marketing strategies that are attractive to youth. All health professionals would agree that smoking cigarettes, or hookah or cigars is one of the most dangerous things you could do to your health. Anything associated with that we would want to try to prevent.”